Adolescence is marked by increased risk behaviors, including substance use and risky sex. Choice impulsivity (CI), or an inability to tolerate delay, s indexed by a preference for small immediate rewards over larger delayed rewards. High levels of CI in adolescents may contribute to their elevated rates of risky behaviors. At the heart of CI in adolescence may be a neurobiological imbalance created by early maturation of reward- sensitive striatal areas and delayed maturation of prefrontal areas, which control other brain regions. This neurobiological imbalance is implicated in increased risk-taking and CI in adolescents and therefore holds great public health significance. However, no existing longitudinal studies examine the maturation of frontal and striatal regions with respect to changes in CI and risky behaviors. This proposed research begins with a K99 project that includes training in behavioral assessments and fMRI data collection, analysis, and interpretation, a baseline behavioral CI assessment, and a baseline fMRI assessment. In the subsequent R00, we propose training in prospective analysis and continued fMRI training, and prospective extensions of fMRI and behavioral CI assessments. Additional annual assessments of CI in adolescents will reveal how CI changes during this developmental period, and how changes in CI are related to changes in real-world risky behaviors. Furthermore, repeated assessment of neural activity in a randomly selected subset of adolescents during CI task performance will reveal how neurodevelopment governs CI and risky behaviors. In the proposed research, CI will be assessed behaviorally each year in the adolescent sample (N=200). Annual assessment of CI will allow for the examination of changes in CI over time, as well as associations of CI with risk behaviors over time. Additionally, neural activity associated with CI will be assessed at two additional time points during the R00 phase in a randomly selected subset of adolescents (N=60) to examine neural activation underlying CI and to characterize neurodevelopment mediating changes in CI. Associations of neural activation with self- reported risky behaviors over time also will be assessed. We hypothesize that there will be a progressive increase in CI in adolescents that will be correlated with an increase in risky behaviors. We also hypothesize that increases in striatal activation and continued diminished levels of prefrontal activation will be associated with increases in CI and risky behaviors. Baseline results will be published in manuscripts at the conclusion of the K99 phase, and prospective behavioral and fMRI results will be presented in published manuscripts by the conclusion of the R00 phase.